Heating elements have been used to fix toner to print media in electrophotographic printing Prior art technology employs one or more resistive heating elements enclosed in a glass bulb which is inserted into a cylinder formed of a thermally conductive material such as aluminum. The exterior surface of the cylinder has a release layer formed from a low adhesion material, such as TEFLON, to reduce toner adhesion to the surface. This embodiment of a fixing device uses a kind of fuser typically referred to as a halogen bulb fuser. The heat generated by the resistive heating element is transferred to the exterior surface of the halogen bulb fuser through radiation, convection and thermal conduction through the wall of the cylinder. Frequently, the glass bulb is filled with a halogen gas to allow the heating element to be operated at a higher temperature.
Another prior art fixing device implementation, using a type of fuser known as an instant on fuser, includes a strip of material forming a resistive heating element. The resistive heating element can be formed on the ceramic substrate through a thick film deposition process. The resistive heating element is covered by a coating of glass. The coating of glass permits low friction rotation of a film sleeve over the glass as well as providing electrical insulation. Typically, in an instant on fuser, the resistive heating element is fabricated on the ceramic substrate with the electrical connections at one end of the long axis of the fuser. Multiple resistive heating elements may be used in the instant on fuser.
A significant technical problem encountered in the use of fixing devices is obtaining an accurate measurement of the temperature on the surface of the fixing device contacting the print media. Generally, in a halogen bulb fuser a single temperature sensor, such as a thermistor, is located near one end and in sliding contact with the halogen bulb fuser outside the path the print media follows as it passes over the halogen bulb fuser. During fixing, print media must pass between the halogen bulb fuser and a pressure roller. In order to permit the print media to pass between the halogen bulb fuser and the pressure roller without the risk of a paper jam, the temperature sensor is typically located on the side of the halogen bulb fuser opposite the region through which the print media passes during fixing. Additionally, in an instant on fuser, the temperature sensor is typically located on the side of the instant on fuser opposite the resistive heating elements in order to eliminate the risk of paper jams. The temperature sensor is part of a circuit which regulates the flow of power to the one or more heating elements within the fixing device in an attempt to establish a uniform temperature profile across the surface of the fixing device. Because the temperature sensor is located relatively remote from the region in which fixing occurs, the difficulty in precisely controlling the fixing temperature is increased.
Contact between the print media and the surface of the fixing device results in a decrease in the surface temperature of the fixing device in those locations on the surface contacting the print media. The temperature sensor provides a measure of the temperature on the surface of the fixing device outside of the print media path in an area which is not thermally loaded. Because of this, an assumption about the surface temperature offset between areas outside of the print media path and areas within the print media path must be made to provide effective control of the fixing device surface temperature in the region contacting the print media. As the width of the print media varies, the value of this temperature offset can change substantially as a result of differences in the thermal loading. This variation in temperature offset increases the difficulty in providing fixing device surface temperatures optimal for fixing toner.
An additional consideration is that the amount of thermal loading from the print media is variable depending upon such print media characteristics as the thermal mass of the print media, the thermal conductivity of the material used for the print media, the surface finish of the print media, and the moisture content of the print media. For example, different thermal loading of the fixing device results from papers having different weights and different surface finishes. Or, between paper of the same weight having different moisture contents, typically as a result of exposure to different humidities, the fixing device is thermally loaded to different degrees. Another factor contributing to the uncertainty of the temperature offset value is the thermal time constant associated with the fixing device and the variable thermal load of the print media. This thermal time constant results in a time varying temperature offset between locations on the surface of the fixing device inside and outside the print media path as the fixing device reaches steady state. These factors create a wide range of variability in the temperature offset between areas of the surface of the fixing device in the path of the print media and the location of the temperature sensor outside of the path of the print media. These types of problems make the use of a temperature sensor outside of the print media path unsuitable for tight control of the fixing device surface temperature.
In past attempts at improving the accuracy of the measurement of the fixing device surface temperature, temperature sensors have been placed in the path of the print media in an attempt to obtain a more accurate measurement of the surface temperature of the fixing device. However, a difficulty encountered in using conventional temperature sensors, such as thermistors, is that, fibers from the print media accumulate on the surface of the sensor and prevent it from obtaining an accurate measure of the fuser surface temperature in the print media path. This difficult has limited the usefulness of locating certain types of temperature sensors in the print media path.
Other attempts at improving the accuracy of the measurement of the fixing device surface temperature have included using temperature sensors that do not contact the surface of the fixing device and are located near the paper path. Although these implementations are less susceptible to paper jams, they are more expensive to implement and they are also susceptible to coating by airborne paper fibers which reduce their ability to reliably measure the surface temperature of the fixing device.
A need exist for an implementation of a fixing device that has the capability to provide a more accurate measurement of the temperature on the surface of the fixing device in the vicinity of the region that contacts the print media for improving control of the fixing device surface temperature while having less sensitivity to the accumulation of the paper fibers and not increasing the likelihood of paper jams.